Glazes are continuous coatings which are usually prepared from fused silicate mixtures and are fusion-bonded to ceramic substrates. A glaze is used on a ceramic substrate to serve one or more of the following purposes: (1) to render the substrate impermeable to liquids and gases, (2) for aesthetic reasons, including covering blemishes and providing decorative effects, (3) to provide protective coatings, and (4) to increase strength.
The exterior portion of a spark plug insulator is exposed to dirt and grease which may result in the formation of an electrically conducting surface and premature failure of the spark plug. Alumina insulator bodies of spark plugs are usually glazed in order to minimize dirt and grease build-up, and to increase the strength and imperviousness of the surface. Depending on the particular properties desired, the glaze can be modified to change the maturing temperature, to add color or to modify the coefficient of thermal expansion.
Glazes applied to alumina substrates must have a low thermal coefficient of expansion, similar to that of the alumina substrate, to avoid undue stresses which can cause spalling, chipping, cracking or crazing of the glaze; from 6 to 7 micro inches per inch per .degree. C. is a typical range of coefficient of thermal expansion for alumina bodies. A glaze with a low coefficient of thermal expansion also strengthens the insulator by inducing compressive stresses at the surface of the glaze-insulator composite. Because glazes involve highly complex multi-component systems, it is difficult to predict the effect of varying or substituting chemical compounds in a glaze formulation, even though general properties of some of the individual components are known. Furthermore, because a glaze is not homogeneous, that is, it may contain one or more dispersed undissolved phases, the ultimate components shown by chemical analysis do not describe a glaze such that the properties are easily predictable.
Because the oxides and carbonates of lead enter into combination with silica and boric acid, lead finds extensive use in glazes. The addition of lead to a glaze lowers the coefficient of thermal expansion and lowers the modulus of elasticity; lead also decreases melt viscosity, reduces the danger of devitrification, broadens the maturing range, and lowers the surface tension of molten glazes, helping to homogenize the glaze and form a defect-free surface.
However, the use of lead compounds in glazes has numerous disadvantages, including a decrease in the abrasion resistance of the glaze, and volatility when fired above cone 6 or 7.
A more serious problem is the toxic nature of the lead compounds used in glazes. Occupational exposure to lead compounds may provide an opportunity for ingestion and subsequent lead extraction by digestive acids. The danger from lead poisoning is amplified because the lead tends to accumulate in the central nervous system of the human body. Increased concern and knowledge relating to environmental health and safety have made it increasingly desirable to substitute a lead-free glaze for lead-containing glazes presently in use. In order to be suitable, lead-free glazes must be non-toxic and contain ingredients which are readily available at a reasonable cost. Transparent glazes over underglaze decorations, are usually preferred.
A lead-free glaze suitable for application to high alumina ceramics has been developed, and is disclosed and claimed in U.S. application Ser. No. 817,194, filed July 20, 1977. This glaze consists essentially of from 50 to 54 percent* SiO.sub.2, from 5 to 8 percent Al.sub.2 O.sub.3, from 6 to 12 percent by weight B.sub.2 O.sub.3, from 4 to 6 percent CaO, from 2 to 8 percent MgO, from 2 to 15 percent by weight BaO, from 5 to 8 percent SrO, from 1 to 2 percent ZnO, and from 4 to 6 percent of a mixture of Na.sub.2 O, K.sub.2 O and Li.sub.2 O. FNT *The terms "percent" and "parts" are used herein and in the appended claims to refer to percent and parts by weight, unless otherwise indicated.